Mesh construction type direct drive fan impedance modeling method considering frequency coupling effect

Abstract

The invention discloses a network construction type direct-driven fan impedance modeling method considering a frequency coupling effect. The method comprises the following steps: constructing a main circuit small signal model of a network construction type direct-driven fan grid-connected system including power grid impedance in a frequency domain; based on a multi-harmonic linearization theory, obtaining an analytical expression between modulation coefficient disturbance and grid-connected point voltage, current and inductive current disturbance signals; combining the main circuit small signal model with the analytical expression to obtain an overall impedance matrix of the grid-connected system of the grid-forming type direct-driven wind turbine generator; using the overall impedance matrix to obtain the overall impedance under the disturbance frequency omega p, and subtracting the impedance component corresponding to the frequency omega p in the power grid impedance matrix from the overall impedance to obtain the equivalent impedance of the grid-forming type direct-driven wind turbine generator. According to the method, the modeling difficulty can be effectively reduced, the accurate and reliable equivalent impedance can be obtained, and technical support is provided for accurately analyzing the stability of the grid-connected system.

Description

Technical Field

[0001] The present invention relates to the field of power generation system stability, and in particular to a grid-type direct-drive wind turbine impedance modeling method taking frequency coupling effects into account. Background Art

[0002] Direct-drive wind turbines are widely used as mainstream models in wind power generation systems due to their mature technology and high cost-effectiveness. However, with the grid connection of a large number of wind turbines, the system inertia and damping will be reduced, which is not conducive to system stability. Therefore, grid-type control technology has been widely valued because it can simulate the inertia and damping characteristics of traditional synchronous generators. In order to better analyze the small signal stability of the grid-connected system of direct-drive wind turbines, it is particularly important to establish a correct impedance model for it.

[0003] At present, in existing studies, when establishing the impedance model of grid-type direct-drive wind turbines, most of them only consider the single-frequency disturbance voltage input and dual-frequency disturbance current output characteristics formed by the asymmetric control structure, and do not involve the mutual coupling between the output dual-frequency components and the power grid, which will lead to inaccurate results when performing stability analysis on the system; secondly, a few studies have established an equivalent impedance model considering frequency coupling based on the obtained VSG output impedance, but the coupling process involved in this method is relatively complicated, which undoubtedly increases the difficulty of system modeling.

[0004] Therefore, a new impedance modeling method for grid-connected direct-drive wind turbines is needed, which can effectively reduce the difficulty of modeling and obtain accurate and reliable equivalent impedance, providing technical support for accurate analysis of the stability of the grid-connected system. Summary of the invention

[0005] In view of this, the purpose of the present invention is to overcome the defects in the prior art and provide a grid-type direct-drive wind turbine impedance modeling method taking into account the frequency coupling effect, which can effectively reduce the difficulty of modeling and obtain accurate and reliable equivalent impedance, thereby providing technical support for accurately analyzing the stability of the grid-connected system.

[0006] The impedance modeling method of a grid-type direct-drive wind turbine taking into account the frequency coupling effect of the present invention comprises:

[0007] Apply voltage disturbance to the grid and construct a main circuit small signal model of the grid-connected direct-drive wind turbine system including grid impedance in the frequency domain.

[0008] Based on the multi-harmonic linearization theory, the analytical expressions between the modulation coefficient disturbance and the grid-connected point voltage, current and inductor current disturbance signals are obtained;

[0009] Combining the main circuit small signal model and analytical expression, the overall impedance matrix Z of the grid-connected direct-drive wind turbine system is obtained. acg ;

[0010] Using the overall impedance matrix Z acg , find the disturbance frequency ω p The overall impedance Z acg (ω p ), the overall impedance Z acg (ω p ) minus the grid impedance matrix Z g Medium p Impedance component Z corresponding to frequency g (ω p ), and the equivalent impedance of the grid-type direct-drive wind turbine is obtained.

[0011] Furthermore, the main circuit small signal model is:

[0012]

[0013] in, and Represent the grid voltage, PCC point voltage, PCC point current and inductor current disturbance signal respectively, represents the modulation coefficient disturbance; Z g , Z L and Z C They represent the grid impedance matrix, filter inductance impedance matrix and filter capacitor impedance matrix respectively, U dc represents the DC voltage steady-state matrix.

[0014] Furthermore, the DC voltage steady-state matrix U dc The expression is:

[0015]

[0016] Among them, U dc Indicates the intermediate DC side voltage of the direct-drive wind turbine system.

[0017] Further, the analytical expression is:

[0018]

[0019] in, and Represent the PCC point voltage, PCC point current and inductor current disturbance signal respectively, represents the modulation coefficient disturbance; Q, P, and R are coefficient matrices.

[0020] Furthermore, the overall impedance matrix Z acg for:

[0021]

[0022] Where E is the identity matrix, Y L is the filter inductance admittance matrix, that is, Y L -1 =Z L .

[0023] Furthermore, the Toeplitz steady-state harmonic matrix is ​​substituted into the main circuit small signal model to eliminate the convolution operation.

[0024] The beneficial effects of the present invention are as follows: the present invention discloses a grid-type direct-drive wind turbine impedance modeling method taking into account the frequency coupling effect, establishes an overall impedance model of the grid-type direct-drive wind turbine grid-connected system based on the multi-harmonic linearization theory, and then obtains an equivalent impedance model of the grid-type direct-drive wind turbine including the frequency coupling effect. The overall modeling method adopted by the present invention only needs to calculate the grid impedance and system output impedance at the disturbance frequency, and there is no need to consider the coupling relationship between the positive and negative sequence impedances during the calculation process. The physical meaning is clearer, and the difficulty of system modeling is effectively reduced; at the same time, modeling based on the multi-harmonic linearization theory will make it so that when the system control structure changes, its impedance model only needs to adjust each coefficient matrix, which is more conducive to the damping characteristics and stability analysis of the grid-type direct-drive wind turbine under the variable control structure than the existing methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention will be further described below in conjunction with the accompanying drawings and embodiments:

[0026] Figure 1 This is the overall structural diagram of the grid-connected system of the grid-building direct-drive wind turbine unit of the present invention;

[0027] Figure 2 This is a virtual synchronous control structure diagram of the grid-side converter in the grid-type direct-drive wind turbine generator system of the present invention;

[0028] Figure 3 This is a schematic diagram of equivalent impedance sweep frequency results of the grid-type direct-drive wind turbine generator system of the present invention;

[0029] Figure 4 It is a schematic flow chart of the impedance modeling method of the grid-type direct-drive wind turbine of the present invention. DETAILED DESCRIPTION

[0030] The present invention is further described below in conjunction with the accompanying drawings, as shown in the drawings:

[0031] This embodiment discloses a grid-type direct-drive wind turbine impedance modeling method taking into account frequency coupling effect, comprising the following steps:

[0032] S1. Apply voltage disturbance to the grid and construct a main circuit small signal model of the grid-connected direct-drive wind turbine system including grid impedance in the frequency domain;

[0033] S2. Based on the multi-harmonic linearization theory, the analytical expressions between the modulation coefficient disturbance and the grid-connected point voltage, current and inductor current disturbance signals are obtained;

[0034] S3. Combine the main circuit small signal model and analytical expression to obtain the overall impedance matrix Z of the grid-connected direct-drive wind turbine system acg ;

[0035] S4. Using the overall impedance matrix Z acg , find the disturbance frequency ω p The overall impedance Z acg (ω p ), the overall impedance Z acg (ω p ) minus the grid impedance matrix Z g Medium p Impedance component Z corresponding to frequency g (ω p ), and the equivalent impedance of the grid-type direct-drive wind turbine is obtained.

[0036] The present invention first establishes a main circuit small signal model in the frequency domain; then, according to the control structure of the wind turbine grid-connected system, an analytical expression between the modulation coefficient and the grid-connected point voltage, current and inductor current disturbance signals is obtained to establish a control circuit small signal model; finally, the overall impedance matrix Z of the wind turbine grid-connected system is obtained by combining the main circuit and the control circuit. acg The grid impedance is considered as a part of the grid-type direct-drive wind turbine, and Z acg The impedance component Z corresponding to the disturbance frequency acg (ω p ) minus the grid impedance matrix Z g The impedance component Z corresponding to the disturbance frequency g (ω p ), and thus the equivalent impedance of the grid-type direct-drive wind turbine is obtained.

[0037] When the impedance modeling of the grid-type direct-drive wind turbine is performed in the present invention, only the grid impedance and the system output impedance at the disturbance frequency need to be calculated, and no complex coupling relationship is involved, which effectively reduces the difficulty of modeling the grid-type direct-drive wind turbine system and can more accurately perform the damping characteristics and stability analysis of the grid-type direct-drive wind turbine under the variable control structure.

[0038] In this embodiment, in step S1, the frequency coupling effect refers to the case where a frequency of ω is injected at the point of common coupling (PCC). p The disturbance voltage will appear in the grid current with positive sequence ω p Frequency and Negative Sequence ω p -2ω 0 The disturbance current is mainly composed of coupling frequency components, where ω0 is the system reference frequency, the system presents a single-input multiple-output feature, and at the same time, ω p -2ω 0 The disturbance current at the coupling frequency will generate a voltage disturbance at the coupling frequency at the PCC point through the grid impedance. The voltage disturbance will further couple the converter to generate ω p Frequency disturbance current.

[0039] Apply voltage disturbance to the grid, according to Figure 1 The system structure shown in the figure is used to establish a mathematical model of the main circuit of the grid-connected direct-drive wind turbine system including the grid impedance in the frequency domain, and the frequency domain main circuit small signal model is obtained by small signal linearization. The main circuit small signal model is:

[0040]

[0041] in, and Represent the grid voltage, PCC point voltage, PCC point current and inductor current disturbance signal respectively, represents the modulation coefficient disturbance; Z g , Z L and Z C They represent the grid impedance matrix, filter inductance impedance matrix and filter capacitor impedance matrix respectively, U dc represents the DC voltage steady-state matrix.

[0042] The DC voltage steady-state matrix U dc The expression is:

[0043]

[0044] Among them, U dc Indicates the intermediate DC side voltage of the direct-drive wind turbine system.

[0045] It should be noted that substituting the Toeplitz steady-state harmonic matrix into the main circuit small signal model can eliminate the convolution operation, thereby obtaining a simplified main circuit small signal model in the frequency domain. Among them, the Toeplitz steady-state harmonic matrix is ​​an existing type of matrix in which the elements on each diagonal are the same.

[0046] In this embodiment, in step S2, according to Figure 2 The virtual synchronous control structure of the grid-side converter in the grid-connected direct-drive wind turbine system shown in the figure obtains the analytical expression between the modulation coefficient and the grid-connected point voltage, current and inductor current disturbance signals: The analytical expression is:

[0047]

[0048] in, and Represent the PCC point voltage, PCC point current and inductor current disturbance signal respectively, represents the modulation coefficient disturbance; Q, P, and R are coefficient matrices.

[0049] The coefficient matrix Q can be solved by the following method: the inductor current disturbance passes through the current loop to obtain the modulation coefficient disturbance signal in the dq coordinate system, and then the A phase modulation coefficient disturbance signal is finally obtained through dq-abc transformation. Thus, the mathematical relationship between the inductor current disturbance and the modulation coefficient disturbance is obtained.

[0050] The coefficient matrix P can be solved according to the following method: the PCC voltage disturbance signal will form an output phase disturbance through the active loop, thereby affecting the dq-abc transformation and the abc-dq transformation. At this time, the steady-state modulation coefficient will affect the modulation coefficient together with the phase disturbance in the dq-abc transformation; secondly, the steady-state current and steady-state voltage of the grid-connected point will affect the modulation coefficient together with the phase disturbance in the abc-dq transformation; finally, the disturbance voltage at the PCC point will affect the modulation coefficient through the voltage loop, and will also pass through the reactive loop to cause the reference voltage to change, thereby affecting the modulation coefficient.

[0051] The coefficient matrix R can be obtained according to the following method: the PCC current disturbance signal will pass through the active loop to form an output phase disturbance, thereby affecting the dq-abc transformation and the abc-dq transformation. At this time, the steady-state modulation coefficient will affect the modulation coefficient with the phase disturbance in the dq-abc transformation; secondly, the steady-state current and steady-state voltage of the PCC point will act with the phase disturbance signal in the abc-dq transformation to affect the modulation coefficient; finally, the disturbance current at the PCC point will also pass through the reactive loop, causing the reference voltage to change, thereby affecting the modulation coefficient.

[0052] In this embodiment, in step S3, the main circuit small signal model and the analytical expression are combined to obtain the overall impedance matrix Z of the grid-connected direct-drive wind turbine system: acg ; The overall impedance matrix Z acg for:

[0053]

[0054] Where E is the identity matrix, Y L is the filter inductance admittance matrix, that is, Y L -1 =Z L .

[0055] In this embodiment, in step S4, the overall impedance matrix Z of the system is used. acg , find the disturbance frequency ω p The overall system impedance Z under acg(ω p ), minus the grid impedance matrix Z g Medium p Impedance component Z corresponding to frequency g (ω p ), thus obtaining the equivalent impedance Z of the grid-type direct-drive wind turbine ac_eq (ω p ), the expression is:

[0056] Z ac_eq (ω p )=Z acg (ω p )-Z g (ω p ).

[0057] In order to verify the correctness of the modeling method in the present invention, a grid-type direct-drive wind turbine simulation model was built in Matlab / Simulink, and the frequency scanning results were compared and verified with the impedance model. Figure 3 The equivalent impedance sweep result of the grid-type direct-drive wind turbine taking into account frequency coupling when the wind speed is 12m / s. The solid line is the equivalent impedance, and the “O” mark is the simulation sweep result. Figure 3 It can be seen that the equivalent impedance established according to the modeling method of the present invention is highly consistent with the simulation scanning result, which verifies the effectiveness and correctness of the modeling method of the present invention.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A grid-type direct-drive wind turbine impedance modeling method taking into account frequency coupling effect, characterized in that: include: Apply voltage disturbance to the grid and construct a main circuit small signal model of the grid-connected direct-drive wind turbine system including grid impedance in the frequency domain. Based on the multi-harmonic linearization theory, the analytical expressions between the modulation coefficient disturbance and the grid-connected point voltage, current and inductor current disturbance signals are obtained; Combining the main circuit small signal model and analytical expression, the overall impedance matrix Z of the grid-connected direct-drive wind turbine system is obtained. acg ; Using the overall impedance matrix Z acg , find the disturbance frequency ω p The overall impedance Z acg (ω p ), the overall impedance Z acg (ω p ) minus the grid impedance matrix Z g Medium p Impedance component Z corresponding to frequency g (ω p ), and the equivalent impedance of the grid-type direct-drive wind turbine is obtained.

2. The impedance modeling method of a grid-type direct-drive wind turbine taking into account frequency coupling effect according to claim 1 is characterized in that: The main circuit small signal model is: in, and Represent the grid voltage, PCC point voltage, PCC point current and inductor current disturbance signal respectively, represents the modulation coefficient disturbance; Z g , Z L and Z C They represent the grid impedance matrix, filter inductance impedance matrix and filter capacitor impedance matrix respectively, U dc represents the DC voltage steady-state matrix.

3. The impedance modeling method of a grid-type direct-drive wind turbine taking into account frequency coupling effect according to claim 2 is characterized in that: The DC voltage steady-state matrix U dc The expression is: Among them, U dc Indicates the intermediate DC side voltage of the direct-drive wind turbine system.

4. The impedance modeling method of a grid-type direct-drive wind turbine taking into account frequency coupling effect according to claim 1 is characterized in that: The analytical expression is: in, and Represent the PCC point voltage, PCC point current and inductor current disturbance signal respectively, represents the modulation coefficient disturbance; Q, P, and R are coefficient matrices.

5. The impedance modeling method of a grid-type direct-drive wind turbine taking into account frequency coupling effect according to claim 1 is characterized in that: The overall impedance matrix Z acg for: Where E is the identity matrix, Y L is the filter inductance admittance matrix, that is, Y L -1 =Z L .

6. The impedance modeling method of a grid-type direct-drive wind turbine taking into account frequency coupling effect according to claim 1 is characterized in that: Substitute the Toeplitz steady-state harmonic matrix into the main circuit small signal model to eliminate the convolution operation.

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